The present invention relates to a developing apparatus employing a magnetic brush developing method.
A conventional mode of the aforesaid developing apparatus provides a developer which is supplied to the surface of a developing sleeve by means of a dispersing roller, said developer being maintained as a magnetic brush by the magnetic intensity of a magnetic roller, the developer being carried to an area opposite (in the developing region) a photosensitive drum via the rotation of a developing sleeve, with the head of the magnetic brush being trimmed by a brush-height regulating member, thus appropriately maintaining the volume of the developer which is carried to the developing region.
Although the magnetic brush formed above the aforesaid developer sleeve becomes thin as it rises in the direction of the normal line along the line of magnetic force opposite the magnetic pole, it overlaps and reaches a high density along the exterior surface of the developing sleeve between the magnetic poles.
Thus, because the volume of developer which is allowed to pass varies greatly with even a slight adjustment error in the positioning of the brush-height regulating member when the leading edge of the brush-height regulating member is provided opposite the thick portion of the magnetic brush, said brush-height regulating member is generally provided opposite the thin portion of the magnetic brush so as to control the volume of carried developer in this area.
The aforesaid developing apparatus possesses the disadvantages hereinafter described.
The magnetic brush becomes abruptly low and thick at a point where there is a slight shifting of position because the range wherein the aforesaid developer becomes thin is exceedingly narrow. When this thick portion of the magnetic brush acquires carrier capability from the developing sleeve, the thin portion of the brush which faces the brush regulating member is depressed and the thin condition is broken.
Therefore, in order to alleviate this type of magnetic brush break down and achieve a proper developer carrier volume, the brush-height regulating gap must necessarily be narrow and adjustment becomes exceedingly difficult.
A concrete numerical example of the aforementioned situation follows. Generally, in giving consideration to factors such as mass production, cost and production technology, the permissible error at the time of assembly becomes a problem at less than .+-.0.05 mm. However, when the necessary brush-height regulating gap is relatively small, for example, Db=0.45 mm, the difference between the upper limit of 0.50 mm and the lower limit of 0.40 mm (e.g., said difference being 0.1 mm) is approximately 22% of the brush-height regulating gap (0.45 mm). When a difference of this magnitude obtains in conventional developing apparatuses, the developing characteristics change markedly, tone reproducability is reduced, and the image becomes hard.
A second disadvantage is that an error of .+-.2.degree. in the positioning accuracy of the magnets provided to the magnetic roller occurs circumferentially from the standpoint of structural assembly, and furthermore, even whenever the magnetic roller is mounted to the developing sleeve and incorporated in a developing apparatus, the same degree of error (.+-.2.degree.) unavoidably occurs.
Thus, when a developing sleeve having a major diameter of 25 mm is used, for example, and the aforesaid error is produced, the position of the magnet center shifts nearly .+-.0.9 mm circumferentially on the surface of the developing sleeve. EQU [(8.degree./360.degree.)33 25 mm.times..pi.=0.89 mm]
Accordingly, when a 2 mm thick sheet metal part is used as the brush-height regulating member and the adjustment position shifts only to the degree described above, the volume of carried developer will change greatly. For example, when the volume of carried developer increases, the developer clogs between the photosensitive drum and the developing sleeve resulting in inferior image quality and damage to the expensive photosensitive drum, and the drive system becomes overloaded due to increased torque which likewise results in damage to the drive system.
A third disadvantage is that because the previously described brush-height regulating gap is essentially very narrow, when developer, paper or other debris aggregate therein causing a narrowing of said regulating gap, the developer ceases to pass only at that section, producing linear whiteouts on the copy image and resulting in damage to the surface of the developing sleeve.